Sterilization is used in a broad range of industrial and medical applications. Sterilization is the complete destruction or irreversible inactivation of all microorganisms. There are many methods for sterilizing, including heat and chemical methods. Heat sterilization is normally done using steam. Some equipment cannot withstand either the heat or the moisture from steam treatment. As a result, chemical sterilization is now commonly used.
Chemical sterilization can be done using alcohols, aldehydes such as formaldehyde, phenols, ozone, ethylene oxide, or hydrogen peroxide. Chemical sterilization does not normally require the use of heat. The method is thus commonly called cold sterilization. Hydrogen peroxide is commonly used today for chemical sterilization.
Use of low concentrations of hydrogen peroxide for chemical sterilization has many advantages. It is easy to handle, can be stored for long periods of time, is noncorrosive, and mixes readily with water. When it decomposes, it forms water and oxygen, nontoxic materials. However, there are problems with using hydrogen peroxide for sterilization. In order to be effective, it must be maintained at a specified concentration. It is therefore normally desirable to maintain as high a concentration as practical during sterilization. Furthermore, hydrogen peroxide will react with some surfaces undergoing sterilization and will also permeate into and through some plastic materials. All of these factors can reduce the concentration of the hydrogen peroxide to levels that make it ineffective at sterilization.
Hydrogen peroxide vapor can condense onto the walls of the sterilization chamber or onto equipment in the chamber. The condensed hydrogen peroxide can potentially degrade or harm the chamber or the equipment.
It is therefore important to be able to determine the concentration of hydrogen peroxide vapor in the sterilization chamber so that enough hydrogen peroxide vapor is present to be effective, but not so much that the hydrogen peroxide vapor damage the equipment.
The concentration of hydrogen peroxide vapor throughout the chamber can vary, because he equipment placed in the chamber can restrict diffusion of sterilant vapor. There may therefore be areas of the chamber which are exposed to higher or lower concentrations of hydrogen peroxide due to these flow restrictions. It is therefore desirable to be able to determine the concentration of hydrogen peroxide in different areas of the sterilization chamber in order to measure the variation in concentration through the sterilization chamber.
There are methods for determining levels of hydrogen peroxide in sterilization chambers. Ando et al. (U.S. Pat. No. 5,608,156) disclose using a semiconductor gas sensor as a means of measuring vapor phase hydrogen peroxide concentrations. The reaction time of the sensor is several tens of seconds, however, and the relation between the sensor output and the concentration of the hydrogen peroxide vapor varies with changes in pressure. Most hydrogen peroxide vapor sterilization procedures involve several treatment steps, usually including at least one step with vacuum. The response of the sensor to hydrogen peroxide through the treatment steps will therefore change, depending on the pressure used in each treatment step.
Cummings (U.S. Pat. No. 4,843,867) discloses a system for determining the concentration of hydrogen peroxide vapor in-situ by simultaneous measurements of two separate properties such as dew point and relative humidity. A microprocessor is then used to fit the two measurements into a model to calculate the hydrogen peroxide concentration. The method uses an indirect approximation based on a number of empirical assumptions, and the accuracy will vary depending on how closely the conditions in the sterilization chamber resemble those used to develop the model.
Van Den Berg et al. (U.S. Pat. No. 5,600,142) disclose a method using near infrared (NIR) spectroscopy to detect hydrogen peroxide vapor. Hydrogen peroxide has an absorption peak at about 1420 nm (nanometers) which can be used to determine its concentration. Water also absorbs in this region, however, and it therefore interferes with the determination of the concentration of hydrogen peroxide. Water is always present when hydrogen peroxide is present, because it is a decomposition product. In order to correct for the interference from water vapor, the water vapor concentration is determined by doing a measurement at remote wavelengths where hydrogen peroxide is transparent. This measured water vapor concentration is used to correct the absorbance at 1420 nm for the contribution due to water. Organic molecules also absorb in this same region, however, and the correction factor for organic molecules depends on the organic compounds which are present. The correction for organic vapors is therefore somewhat subjective, because one does not normally know what organics are present.
The NIR method requires doing measurements at two different wavelengths and making corrections for the presence of water vapor, organics, or both. The electronic equipment for doing these corrections is complex and expensive, and the correction for the presence of organic compounds is subjective.
There is a need for a method of determining the concentration of hydrogen peroxide vapor or gas that is not dependent on correcting for the presence of water vapor and organic molecules. There is also a need for a method of measuring hydrogen peroxide that does not require the use of expensive electronics, such as those which do measurements at two different wavelengths and apply complex correction factors.